The present invention relates in general to blood circulatory assist devices, and, more specifically, to autonomous control of a pump to maintain optimum blood flow under a variety of conditions including partial obstructions and low blood volume.
Many types of circulatory assist devices are available to either short term or long term support for patients having cardiovascular disease. For example, a heart pump system known as a left ventricular assist device (LVAD) can provide long term patient support with an implantable pump associated with an externally-worn pump control unit and batteries. The LVAD improves circulation throughout the body by assisting the left side of the heart in pumping blood. One such system is the DuraHeart® LVAS system made by Terumo Heart, Inc., of Ann Arbor, Mich. One embodiment of the DuraHeart® system may employ a centrifugal pump with a magnetically levitated impeller to pump blood from the left ventricle to the aorta. An electric motor magnetically coupled to the impeller is driven at a speed appropriate to obtain the desired blood flow through the pump.
A typical cardiac assist system includes a pumping unit, electrical motor (e.g., a brushless DC motor integrated into the pump), drive electronics, microprocessor control unit, and an energy source such as rechargeable batteries. The system may be implantable, either fully or partially. The goal of the control unit is to autonomously control the pump performance to satisfy the physiologic needs of the patient while maintaining safe and reliable system operation. A control system for varying pump speed to achieve a target blood flow based on physiologic conditions is shown in U.S. Pat. No. 7,160,243, issued Jan. 9, 2007, which is incorporated herein by reference in its entirety. Thus, a target blood flow rate may be established based on the patient's heart rate so that the physiologic demand is met. The control unit may establish a speed setpoint for the pump motor to achieve the target flow. Whether the control unit controls the speed setpoint in order to achieve flow on demand or whether a pump speed is merely controlled to achieve a static flow or speed as determined separately by a physician, it is essential to automatically monitor pump performance to ensure that life support functions are maintained.
The actual blood flow being delivered to the patient by the assist device can be monitored either directly by sensors or indirectly by inferring flow based on motor current and speed. Despite the attempt by the control unit to maintain a target flow, various conditions such as obstructions of the inflow conduit or outflow conduit from the pump, low blood volume due to dehydrations, or other problems may cause the blood flow to decrease. Low flow and no flow alarms are conventionally employed to indicate conditions when the blood flow through the pump has inadvertently fallen below a low flow threshold or a no flow threshold, respectively. The alarms may comprise warning sounds, lights, or messages to allow the patient or caregiver to take corrective action. In order to provide a greater safety margin, it would be desirable to identify and correct flow problems before the low flow or no flow thresholds are reached.